Producers of audiovisual content, such as television broadcasters or advertisers, are often interested in having knowledge as to whether, where and when their work is distributed. To this end, “broadcast monitoring” systems have been developed recently.
In one known broadcast monitoring method, sometimes referred to as active monitoring, a watermark is embedded in the video content. The payload in the watermark is a pointer to a database entry corresponding to the video sequence (e.g. a clip, a movie scene, a commercial). The watermark is retrieved and used to identify the content. A problem of this method is that a large payload is necessary for exploiting a large database. Such a large payload is difficult to embed in an imperceptible and unobtrusive manner.
In another known broadcast monitoring method, referred to as passive monitoring, so-called robust features are extracted from the content. Robust features are variables computed from the content, which remain more or less unchanged as long as the scene does not change too much. For every scene there is a unique set of robust features. For example, video pictures are divided into blocks. Each block is represented by a bit indicating whether the luminance of said block is higher or lower than a given reference value (e.g. the luminance of the neighboring block). The bit string corresponding to the robust features is then used as a signature for the particular scene in the database. A problem of this method is that the bit string extracted at the monitoring end may be slightly different from the signature in the database. This makes it difficult to search the signature in the database which most closely resembles the extracted bit string. Moreover, the signature is not necessarily unique.